The use of gas-filled Geiger Mueller tubes for radiation detection is well known. In operating condition when no incoming radiation is present, the tube is not conducting. At incoming radiation the tube starts to conduct and continues to do so even if the radiation disappears. The current is producing ions and free electrons in the gas. To stop the current from flowing, the applied voltage over the tube must be reduced under a certain treshold value, the quenching voltage. This is obtained two times every cycle at AC excitation and with passive or active circuitry at DC excitation. The voltage reduction must be of sufficient time to get the free electrons and the ions neutralized. If normal operating voltage is applied too soon over the electrodes after the discharge has been stopped, the tube will start to conduct again, even if there is no incoming radiation. The most used quenching circuit for DC operation is of a passive CR type, often used by tube manufacturers in datasheets to specify the sensitivity of the tube. FIG. 1 shows a diagram of this CR quenching circuit and FIG. 2 exemplifies the voltage over and the current flowing through the tube.
These type of tubes are frequently used as primary detectors in systems made for radiation detection, where radiation normally is not present. The systems shall indicate when the radiation increases to a level or with a rate that require some action, for example monitoring nuclear leakage or radiation from a fire. False triggering is not acceptable in such systems.
At no incoming radiation, from the source that is intended to be surveyed, spurious discharges can occur from stray photons, beta or gamma rays, static discharges, inpurities in cathode material and the like. This is noise to the system. The discharge pulses from this background noise are distributed at random time. Therefor it is possible to have a number of pulses within a short time even at very low mean pulse rate. For example, at a mean pulse rate of one pulse per minute, the probability of having three pulses in one second is one in 15 days, and the probability of having 5 pulses in one second is one in 3000 years. This is the reason why no known systems are allowed to react for just a few pulses. If you should maintain a good margin against false triggering, most systems require at least 5 to 10 pulses under a limited time to take action. The pulse rate increases for increasing incoming radiation up to a saturation pulse rate of about 170 Hz for an ordinary ultra violet detector tube with CR quenching circuits. This gives a low limit time of 5/170 s (29 ms) to the ultra violet flame detecting system with the CR system described above, at high radiation levels. The invention has means to have a pulse rate of 1700 Hz which gives a low limit radiation detecting time of 3 ms. U.S. Pat. No. 4,162,425 discloses have a modification of the CR quenching circuit which increases the saturation pulse rate with less than two times.